Method of coating using a bead coater



Feb. 25, 1969 J. J. MORIARTY 3,429,741

METHOD OF COATING USING A BEAD COATER Filed June 11, 1965 utnaus J.NOR/ART) INVENTOR. 1 BY W w 9M4? (9 1.

ATTORNEYS United States Patent METHOD OF COATING USING A BEAD COATERJerome J. Moriarty, Rochester, N.Y., assignor to Eastman Kodak Company,Rochester, N .Y., a corporation of New Jersey Filed June 11, 1965, Ser.No. 463,155

US. Cl. 117111 Int. Cl. B05c 5/02, 3/18, 1/12 2 Claims ABSTRACT OF THEDISCLOSURE The present invention relates to a continuous coatingmechanism and more particularly to a single side coating mechanism fortransferring a liquid layer to a continuous web.

In the art of continuous coating, several mechanisms are known, with onebeing termed a bead coater. In a bead coating mechanism, a continuousweb is passed over a coating tank having therein a driven lift rollerspaced a few thousandths of an inch below the web and which raises theliquid to contact the moving web, with the contacting liquid beingreferred to as a bead. The lift roller has a surface veocity opposed tothat of the web. Thus the liquid bead is sheared during the coatingprocess. As the speed of these bead coaters is increased, it sometimesoccurs that the bead tends to be shifted laterally or otherwise deformedbecause of the various stresses in the liquid. At certain speeds thisshifting tends to make the bead unstable or vibratory or to capturebubbles so that the coating has varying thicknesses throughout thelength of the web. Probably this shifting causing such variations iswhat might be termed a harmonic vibration of the liquid bead. Suchvariations in the coating, particularly in the art of manufacture ofphotosensitive films, often cause the finished product to beunacceptable for marketing.

Therefore, an object of the present invention is to provide an improvedcontinuous bead coating mechanism.

In accordance with one embodiment of my invention, a web is passed overa coating liquid tank with the liquid being raised at a constant ratefrom the tank by a liquid lift roller having a surface velocit oppositethat of the web. The bead of liquid formed between the lift roller andthe web has immersed therein a tensioned wire which dampens any tendencyof the bead to become unstable.

The subject matter which is regarded as my invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. The invention, however, as to its organization andoperation, together with further objects and advantages thereof willbest be understood by reference to the following description taken inconnection with the accompanying drawing, in which:

FIG. 1 is an elevation view illustrating the general layout of a beadcoater; and

FIG. 2 is a cross-section view taken along the line 2-2 of FIG. 1 andillustrating One embodiment of my invention.

Referring now to the drawing, wherein like numbers indicate similarparts, a web such as a cellulosic film base material is passed over asupport and transport roller 11 to come close to a tank 12 havingtherein a coating liquid 13. Partially submerged in the coating liquid13 is a lift roller 14 which raises continuousy the liquid in a stream15 from the tank 12. As indicated by the arrows 16, the web transportroll 11 passes the web 10 toward the left over the tank 12 (FIG. 2). Asindicated by the arrow 17, the lift roller 14 causes the liquid to movein the opposite direction at the surface closest to the web 10. Thisrelative rotation of the rollers 11 and 14 is developed by gearing in agear box 18 and driven at controllable speeds by a motor 19 shown inFIG. 1.

The continuous lift of the liquid 13 causes a bead 21 of liquid to beformed in the nip between the lift roller 14 and the web 10. A portionof liquid stream 15 flowing to the bead 21 is transferred to the web 10as a coating layer 22. The excess liquid of the bead 21 returns to thetank 12 as indicated at 23. Since the liquid viscosity, relativevelocities (16 and 17), and nip spacing of the system may be variedconsiderably, accurate and uniform coatings 22 may be developed by thistype of coating system. Also, these system parameters may be adjusted toprovide maximum practical flow of liquid 15 into the head 21 whilemaintaining a stable condition to operate at relatively high coatingspeeds. For practical purposes the minimum nip clearance is usuall about10 mils. Maximum clearance, depending on adhesive and cohesiveproperties of the liquid 13, is reached when the bead 21 breaks orbecomes unstable.

However, at certain higher speeds of operation of this coating processand depending on the properties of the liquid 13, the bead 21 tends toexhibit instability characteristics. This instability causesnonuniformities in the coating layer 22. Since the layer must attack andswell the cellulosic base web 10, any nonuniformities cause differentialattack and swell of the web. Such differentials result in physical and/or optical defects after the coating layer has dried. Reducing the nipclearance toward minimum tends to maintain a stable bead 21 but onceminimum clearance is reached, attempts to increase the bead coatingprocess velocity have been thwarted in prior art devices using certaincoating liquids.

In accordance with my invention, this instability is overcome by placingwithin the bead 21 a taut rod or wire 24. The wire 24 may be tightenedin place as by lock nuts 25. Apparently the wire 24 dampens anytendencies of the bead 21 to vibrate laterally or to develop bubblesbecause of the high speed shear action resulting from the relativevelocities of the web 10 and the liquid being lifted.

In one example of the operation of my invention, a 42" wide transparentcellulose acetate web 10 was transported over the tank 12 at seven feetper minute to be coated with a liquid composition including dispersedgelatin in organic solvents and having a viscosity of 1 centipoise. Thelift roller 14 was operated at surface speeds of about two to twelvefeet per minute with a nip clear ance of 0.020". Without the wire 24,bubbles were present in the bead at nodal points, and the resultingcoating had nonuniformities. Normal adjustments of nip spacing,temperature regulation etc. did not correct the situation. When the tautwire 24 of about 9 or 12 mils 316 stainless steel was introduced intothe bead 21, all nodes in the bead disappeared. Thus, I have found thata coating having reduced stability characteristics may now be applied athigher speeds by the bead coating process because of the stabilizationafforded by the taut wire 24.

Empirical data indicates that the taut wire 24 is most effective ifplaced nearest the up-stream 15 and spaced from the web 10 a distancegreater than the desired thickness of the layer 22. When so placed, nononuniformities in the coating layer 22 were noted even at web speeds upto forty-five feet per minute and surface speeds of the lift roller 14of between five and thirty feet per minute. However, the overall shapeof the head is changed slightly by the taut wire 24 Apparently, thischange is caused by its interruption of the up-stream 15. Also it shouldbe noted that except for possible abrasive properties in the fluid 13,there is no wearing of the wire 24 so that even a small diameter wirewill not cause appreciable maintenance problems.

I claim:

1. The bead coating method of applying a smooth layer of a coatingliquid onto a web surface at high speeds and using a lift roller spacedfrom said web surface by a distance greater than the thickness of thelayer applied to said web to maintain the bead, comprising the steps of:

moving the web surface to be coated in transfer relation with said liftroller;

raising a layer of liquid from a liquid supply on the surface of saidlift roller to form and thereafter maintain a continuous bead of liquidin the space between the web surface and the lift roller, said raisingstep providing a liquid flow generally opposite to the motion of theweb; and

internally stabilizing said bead to provide a uniformly thick layer ofcoating on the web.

2. The method of head coating according to claim 1, characterized inthat the internally stabilizing step is accomplished by a thin, rigidobstruction extending the full length of said head in spaced relation tothe web and said lift roller and entirely immersed in the bead.

References Cited UNITED STATES PATENTS 3,000,760 9/1961 Greiller 117-111ALFRED L. LEAVITT, Primary Examiner.

CHARLES R. WILSON, Assistant Examiner.

US. Cl. X.R. 1l7120

